If units of a variable factor are increasingly added to a fixed factor and the marginal physical product keeps increasing, production is said to be taking p...

The production process described in the question, where the marginal physical product of the variable factor increases as more units of it are added to a fixed factor, is referred to as increasing returns to the variable factor. In this case, the variable factor is becoming increasingly productive as more units of it are added, which results in an increase in the marginal physical product. Increasing returns to scale, on the other hand, refer to the situation where an increase in all factors of production leads to a more than proportionate increase in output. Constant returns to the variable factor occur when increasing the variable factor by a certain percentage leads to a proportional increase in output. Finally, external economies of scale refer to the cost advantages that a firm experiences as a result of factors outside of its control, such as the growth of an industry or the development of infrastructure.